The present invention relates to an exposure apparatus for manufacturing devices such as semiconductor elements and liquid crystal elements.
FIG. 2 shows the schematic arrangement of a conventional semiconductor exposure apparatus. Referring to FIG. 2, reference numeral 10 denotes an illumination optical unit; 11, a reticle (original plate); 12, a reticle stage; 13, a projection optical unit; 14, a wafer (exposure target); and 15, a wafer stage. The illumination optical unit 10 includes an exposure light source. As the light source, a mercury lamp, a KrF excimer laser, or the like is used. A circuit pattern to be exposed is drawn on the glass surface of the reticle 11. The reticle 11 is mounted on the reticle stage 12. The pattern of the reticle 11 is to be transferred to the wafer 14. The wafer stage 15 can move with the wafer 14 being mounted on it.
The operation of the exposure apparatus will be described. Light from the illumination optical unit 10 is guided to the reticle 11 positioned by the reticle stage 12, is transmitted through the projection optical unit 13, and is transferred (exposed) onto the wafer 14. In the exposure apparatus, the wafer stage 15 performs positioning at a desired position within a plane. When the above exposure is repeated, the pattern of the reticle 11 is exposed onto the wafer 14 a plurality of times.
Usually, a semiconductor element is fabricated not by only exposure using one reticle 11, but by performing exposure a plurality of times in an overlapping manner on the same wafer while exchanging the reticle 11. For this purpose, the wafer once exposed must be mounted on the wafer stage 15 again and be precisely aligned with the reticle 11. The semiconductor exposure apparatus has an aligning function for this. According to a typical method, an alignment mark used for alignment is exposed in advance, and this alignment mark is observed with a microscope 16, thereby performing alignment.
In both exposure and alignment described above, the positions of the reticle stage 12 and wafer stage 15 must be held precisely. If a positioning error is large, the circuit pattern is transferred to be displaced from the pattern on the reticle, so desired circuit characteristics cannot be obtained. The same inconvenience also occurs when a positioning error occurs during alignment. Therefore, the positioning performance of the two stages is a significant performance index directly associated with the performance of the exposure apparatus.
Each stage is positioned within the plane by positioning control performed by a position measurement unit 17 using a laser interferometer or the like. FIG. 3 is an illustration of a positioning control unit. The positioning control unit is constituted by a feedback loop. In the feedback loop, a difference between position information x obtained by the position measurement unit 17 and a target position r is calculated. The calculated difference is subjected to an appropriate compensating calculation by a compensator 18 to form a drive instruction value for the wafer stage 15. Along with the recent development of microprocessors, this feedback loop is often constituted by a software servo mainly including a digital filter.
Generally, the measurement axis of the position measurement unit 17 is arranged to coincide with the optical central position constituted by the projection optical unit 13, i.e., the exposure center. This is because the exposed position must be held precisely. In other words, for example, the wafer stage 15 is controlled on a coordinate system having an exposure central position as the reference.
A case wherein a disturbance is applied to the wafer stage 15 will be considered. For example, the vibration of the building where the exposure apparatus is installed, or a reaction produced when the wafer stage 15 itself moves, may excite the main body structure on which the wafer stage is mounted (i.e., the position measurement unit 17). Such a disturbance includes not only a component of one direction but also a translational component and a rotational component. Even if only a translational component is applied as the disturbance, a disturbance in a rotational direction is sometimes excited as another component. Inversely, even if only a rotational component is applied as the disturbance, a disturbance in a translational direction is sometimes excited as another component.
When these disturbance components are generated, the wafer stage 15 operates to suppress them with the feedback loop. According to the basic performance of the exposure apparatus, a sufficiently high positioning performance must be guaranteed even when a disturbance of a certain degree is applied.
As described above, the disturbance is suppressed merely with reference to the exposure center. Accordingly, a sufficiently high positioning performance is guaranteed only from the viewpoint of the exposure central position.
Concerning alignment, it is often performed through observation of an alignment mark on an axis not coinciding with the exposure center, as described in the prior art. In this case, the alignment precision is regulated by the positioning performance with reference to the alignment mark observation position.
A case wherein a disturbance in a rotational direction is applied during alignment will be considered. Even if a sufficiently high positioning precision is obtained with reference to the exposure central position, it cannot be necessarily obtained at the mark observation position. This is because of the following reason. Assume that a positioning precision in a rotational direction at the exposure central position is guaranteed within a target value .+-.0.1 ppm. If the distance between the exposure central position and the mark observation position is 50 mm, since the rotational angle is small, an approximate calculation reveals that an alignment error or an error from an observation result of 50.times.0.1.times.10.sup.-6 =5 nm occurs. In this manner, to guarantee a sufficiently high positioning precision on a coordinate system having the exposure center as the reference does not directly guarantee completely the same precision in alignment, leading to a problem.